Analysis of CyberKnife intracranial treatment plans using ICRU 91 dose reporting: A retrospective study

Abstract

ICRU 91, published in 2017, is an international standard for prescribing, recording, and reporting stereotactic treatments. Since its release, there has been limited research published on the implementation and impact of ICRU 91 on clinical practice. This work provides an assessment of the recommended ICRU 91 dose reporting metrics for their use in clinical treatment planning. A set of 180 intracranial stereotactic treatment plans for patients treated by the CyberKnife (CK) system were analyzed retrospectively using the ICRU 91 reporting metrics. The 180 plans comprised 60 trigeminal neuralgia (TGN), 60 meningioma (MEN), and 60 acoustic neuroma (AN) cases. The reporting metrics included the planning target volume (PTV) near-minimum dose ( $D_{\mathrm{near}-\min }$ ), near-maximum dose ( $D_{\mathrm{near}-\max }$ ), and median dose ( $D_{50\%}$ ), as well as the gradient index (GI) and conformity index (CI). The metrics were assessed for statistical correlation with several treatment plan parameters. In the TGN plan group, owing to the small targets, $D_{\mathrm{near}-\min }$ was greater than $D_{\mathrm{near}-\max }$ in 42 plans, whereas both metrics were not applicable in 17 plans. The $D_{50\%}$ metric was predominantly influenced by the prescription isodose line (PIDL). The GI was significantly dependent on target volume in all analyses performed, where the variables were inversely related. The CI was only dependent on target volume in treatment plans for small targets. The ICRU 91 $D_{\mathrm{near}-\min }$ and $D_{\mathrm{near}-\max }$ metrics breakdown in plans for small target volumes below 1 cm³ ; the Min and Max pixel should be reported in such cases. The $D_{50\%}$ metric is of limited use for treatment planning. Given their volume dependence, the GI and CI metrics could potentially serve as plan evaluation tools in the planning of the sites analyzed in this study, which would ultimately improve treatment plan quality.

Keywords: CyberKnife; ICRU 91; conformity index; gradient index; stereotactic; treatment planning.